The present invention relates to an electrochromic display element comprising as an electrochromic material a transition metal oxide such as tungsten oxide. More particularly, the present invention relates to an improvement in a counter electrode in such electrochromic display element.
An electrochromic display element ordinarily has a structure as shown in the sectional view of FIG. 1. In FIG. 1, referential numeral 1 represents a front glass substrate, reference numeral 2 represents a transparent conductive layer comprised of In.sub.2 O.sub.3 -SnO.sub.2, SnO.sub.2 or the like, and reference numeral 3 represents an electrochromic material comprised of a transition metal oxide such as WO.sub.3, MoO.sub.3 or V.sub.2 O.sub.5. Reference numeral 4 represents a passivation film for the transparent conductive layer 2 and the film 4 is comprised of SiO, SiO.sub.2 or the like. Reference numeral 5 represents a liquid electrolyte comprised of a solution of a lithium salt in an organic compound as a solvent, such as a propylene carbonate solution of LiClO.sub.4. Reference numerals 6 and 7 represent a counter electrode and a back glass substrate, respectively. A reflecting material such as a white powder of titanium oxide is incorporated in the liquid electrolyte 5. Reference numerals 9 and 10 represent a conductor and a gasket, respectively. A filmy conductor is illustrated as the conductor 9 in the drawings, but it may be a meshlike wire conductor.
As the counter electrode in such conventional electrochromic display element, there has broadly been used a counter electrode of Li.sub.x WO.sub.3 prepared by forming a tungsten oxide film on a transparent electrode on the back glass substrate and injecting Li ions into this tungsten oxide film prior to formation of a cell. This counter electrode, however, has the following defects.
If a segment having a display electrode is colored, Li is dissolved in the liquid electrolyte in the counter electrode, and the electrode of Li.sub.x WO.sub.3 is changed to Li.sub.x-.delta. WO.sub.3 (.delta.&gt;0) by this coloration. If the other segment is then colored, the electrode is changed to Li.sub.x-.delta.-.delta.' WO.sub.3 (.delta.'&gt;0). In this case, a common electrode is used for the counter electrode. As is seen from the electrochemical consideration, the potential of the counter electrode depends on the Li content in the electrode. Accordingly, if the constant potential driving method, which is a preferred driving method, is adopted, the potential substantially applied to the display electrode is changed depending on whether the composition of the counter electrode is Li.sub.x WO.sub.3, Li.sub.x-.delta. WO.sub.3 or Li.sub.x-.delta.-.delta.' WO.sub.3. This causes a non-uniform brightness among colored segments and this phenomenon is not desired.
As means for eliminating such non-uniform brightness, there may be considered a method in which the thickness of Li.sub.x WO.sub.3 of the counter electrode is increased so that the value of .delta. or .delta.' is relatively reduced. However, increase of the thickness of Li.sub.x WO.sub.3 results in increase of the resistance of the Li.sub.x WO.sub.3 layer, and another defect of reduction of the response speed is brought about. From the practical viewpoint, use of a layer of Li.sub.x WO.sub.3 having a thickness exceeding 5000 A is not permissible.
The following references are cited to show the state of the art:
(i) Japanese Patent Application Laid-Open Specification No. 8983/72 PA1 (ii) Japanese Patent Application Laid-Open Specification No. 13891/72 PA1 (iii) Specification of U.S. Pat. No. 3,982,472